1. Field of the Invention
The present invention relates to a light shielding device, used for a lens (e.g., a photographic lens) including a lens barrel movable in an optical axis direction, for preventing ambient light from entering the lens through a clearance around the movable lens barrel.
2. Description of the Prior Art
In extendable lenses (e.g., retractable lenses or telescopic lenses) for cameras in which at least one movable lens barrel (movable barrel) is moved in an optical axis direction, a clearance must be secured between the movable lens barrel and an outer member (e.g., an outer barrel or a frame of a camera) provided around the movable lens barrel to allow a smooth advancing operation and a smooth retracting operation (these operations will be hereinafter collectively referred to as a lens advancing/retracting operation). Although providing the extendable lens with such a clearance prevents this interference from occurring and makes the smooth and swift advancing/retracting operation of the extendable lens possible, ambient light may enter inside the extendable lens through the clearance and reach a light receiving surface (e.g., a sensitive surface of film or an image pickup device) of the camera, thus interfering with proper picture taking.
To prevent such an entry of light, a light shielding device has been proposed. This conventional light shielding device is provided with a light shield ring made out of a sheet material of rubber, synthetic leather or the like, wherein a radially outer portion of the light shield ring is fixed to the aforementioned outer member by a fixing device such as an adhesive, while a radially inner portion of the light shield ring is made to be fitted on the outer peripheral surface of the movable lens barrel to prevent ambient light from entering the extendable lens through a clearance between the movable lens barrel and the outer member by the contact of the light shield ring with the outer peripheral surface of the movable lens barrel even if the movable lens barrel moves in the optical axis direction relative to the outer member. However, this sort of light shielding device requires a process of fixing the light shield ring, which is small in thickness and radial dimensions, to the outer member by a fixing member such as an adhesive so that the light shield ring is positioned coaxially with the movable lens barrel during assembly of the extendable lens. This troublesome process considerably impedes the assembling procedure of the extendable lens. Specifically, it is difficult to install the light shield ring coaxially with the movable lens barrel, so that the light shield ring is often installed eccentrically. This eccentric installation of the light shield ring deteriorates an original purpose of the light shield ring, i.e., the light-shielding ability of the light shield ring.
To prevent this from occurring, a technique of closing the clearance between the outer peripheral surface of the movable lens barrel and the outer member (an outer barrel or a frame of a camera) in a light-tight fashion by a similar light shield ring which is slidably fitted on the outer peripheral surface of the movable lens barrel has been proposed in Japanese Laid-Open Patent Publication (Kokai) No. 2001-337257. This technique can be advantageously used to facilitate the assembling procedure of the camera since the light shield ring only needs to be fitted on the outer peripheral surface of the movable lens barrel. Additionally, this technique advantageously prevents the light shield ring from becoming eccentric to the movable lens barrel since the light shield ring is fitted on the movable lens barrel to be held coaxially with the outer peripheral surface of the movable lens barrel.
In JLPP No. 2001-337257, the light shield ring needs to be held on the movable lens barrel with a required fitting force to maintain the fit between the light shield ring and the movable lens barrel because the light shield ring is inserted into a camera frame to be installed therein in a floating state Therefore, when the extendable lens operates by moving the movable lens barrel forward and rearward, the frictional force produced by the fit between the outer peripheral surface of the movable lens barrel and the light shield ring makes it difficult for the movable lens barrel to move smoothly and lightly, which causes an increased load on a lens driven mechanism thereof. If the fitting force of the light shield ring exerted on the movable lens barrel is weakened to prevent this problem from occurring, the light shield ring can be easily moved in radial directions relative to the movable lens barrel. Furthermore, the radially outer portion of the light shield ring is not fixed, and accordingly, the position of the light shield ring is not fixed inside of the camera frame. Therefore, the light shield ring easily becomes eccentric to the movable lens group. If the light shield ring becomes eccentric to the movable lens barrel, a circumferential portion of the radially outer portion of the light shield ring which is decentered radially outwards decreases in light-shielding ability and another circumferential portion of the radially outer portion of the light shield ring which is decentered radially inwards becomes easily pulled out by the lens advancing/retracting operation of the movable lens barrel. If the light shield ring is partly pulled out of the camera frame, the radially inner portion of the light shield ring is partly exposed to the outside of the camera frame along the outer peripheral surface of the movable lens barrel, which deteriorates the outward appearance of the camera. Although a stiffening ring is fixed to the radially outer portion of the light shield ring by a fixing device such as an adhesive to increase stiffness of the light shield ring in JLPP No. 2001-337257, it is difficult to prevent the light shield ring from being pulled out of the camera frame even by the installation of the stiffening ring because a radially inner portion of the light shield ring increases in fitting force exerted on the movable lens barrel to thereby make this portion easy to pull out of its initial position when the stiffening ring becomes eccentric. Additionally, although it is possible to increase the length of the camera frame in the optical axis direction to prevent the light shield ring from being exposed to the outside of the camera frame even if the light shield ring is pulled out of the camera body, an increase in length of the camera frame increases the thickness of the camera in the optical axis direction, which becomes an obstacle to a reduction in thickness and miniaturization of the camera.